Electric utility grids have traditionally been supplied by large centralized power plants. These large power plants are interconnected by high capacity transmission lines that provide improved grid stability, increased fault tolerance, and increased flexibility in load profile management. In these traditional systems, power is distributed outward from the centralized power plants through a hierarchy of power line networks to multiple points of usage.
The increasing use of renewable power sources, however, has introduced distributed generation (DG) capacity to power grids. DG power sources may be located anywhere on the grid, typically close to a local power load. Examples of DG power sources include photovoltaic (PV) panels and wind turbines which are scattered at customer locations throughout the grid. DG sources can vary in size from a few watts to several mega-watts. The amount of power available from these sources can vary based on various factors such as available solar irradiation and wind speeds. At certain times, all of the power from these sources may be consumed by local building loads, while at other times, excess power is fed back into the grid. Thus, utility grids have become complex, interconnected structures with power flowing in multiple directions depending on the availability of power from multiple sources and demand from multiple loads at any specific time.
“Islanding” is a condition in which a portion of a grid containing some power generation capacity and some amount of load becomes isolated from the remainder of the grid, but continues to operate independently because the phase-locked loop (PLL) or other synchronization functionality continues to provide a reference for the power flowing in the isolated portion of the grid. Depending on the specific conditions, islanding may be problematic because the local power generation capacity loses synchronization with the grid. Moreover, when an islanding condition is detected, an assumption is typically made that there is no point in harvesting power from the local power generation capacity because the power cannot be fed back into the grid. Thus, if an islanding condition is detected, the local power generation capacity is disabled and disconnected from the grid. This is referred to as anti-islanding (AI) protection, and the detection of islanding conditions is an ongoing challenge.